This invention generally relates to a system and method for using an atmospheric plasma in the processing of substrates and particularly to the fabrication of semiconductor devices. More specifically this invention relates to a method and system to regulate the position of a plasma jet used to produce an intense hot gas stream employed in the manufacture of semiconductor devices such as Miniature Electro-Mechanical Systems (MEMS).
An atmospheric arc-type plasma referred to as a plasma jet may be used for processing substrates used in the manufacture of semiconductor devices. An atmospheric, plasma jet generation system has previously been described; see the International patent WO9746056, by Siniaguine, entitled xe2x80x9cApparatus for generating and deflecting a plasma jet.xe2x80x9d This atmospheric plasma provides a means for generating an intense, hot gas stream that subsequently can be used for processing a substrate. Processing of the substrate may include: etching by reactive species generated in the hot gas stream; deposition resulting from species introduced into the gas stream; and thermal processing of the substrate by the heat flux carried to the substrate by the hot gas stream. An etching application for polymer removal, has previously been disclosed in the prior International patent pending application no. PCT/US00/27113 by Bollinger and Tokmouline and which is entitled: xe2x80x9cAtmospheric process and system for controlled, rapid removal of polymers from high depth to width aspect ratio holes.xe2x80x9d The application of very rapid thermal processing has been disclosed in International patent pending Application No. PCT/US00/41492 by Bollinger and Tokmouline xe2x80x9cMethod for rapid thermal processing of substrates.xe2x80x9d
This atmospheric plasma as used in these prior international patent applications is shown in FIG. 1 and is also described in the aforementioned patent applications by Bollinger and Tokmouline. In such system, an arc-type electrical discharge or plasma jet 20 is generated between two electrode subassemblies, an anode 22 and a cathode 24. The geometric configuration of the generated plasma jet 20 is such that it creates a vertex 25 where an intense hot gas stream 26 is produced that is directed toward a substrate or wafer 28 that is to be processed by the stream 26. The substrate 28 is held in a substrate holder 30 that is moved through the hot gas stream. Control and repeatability of the hot gas stream 26 is critical for highly controlled, repeatable treatment of the substrate.
Arc-type plasmas by their nature involve electrical current flow and are well known for unstable behavior. Stabilization using this type of plasma generation system is accomplished in the prior art as illustrated in FIG. 2 by an optical sensing of the plasma jet position at P with optical sensors such as a CCD camera. These generate an image into a computer, which produces a feedback signal, not shown, to a steering magnetic field M. The magnetic field M is intended to correct for a sensed deviation in the plasma jet path position as shown from the solid line at 20 to the dashed line at 20xe2x80x2. In the prior art the adjustment possible with a single magnetic field does not lend itself for compensating for both a positional and angular alignment of the jet at the vertex 25. Hence, the repeatability of a process as measured by uniformity of treatment by gas stream 26, U%, has been limited to approximately 5%.
%U=100xc3x97(max. treatmentxe2x88x92min. treatment)/(ave. treatment)xe2x80x83xe2x80x83(1)
Where xe2x80x9ctreatmentxe2x80x9d refers to a measured process result depending on the application, such as etch depth in an etch application or diffusion depth of a doping impurity in a thermal processing application. For some applications, such as for stripping photoresist from a wafer in which the etch selectivity to the layer underlying the photoresist is practically infinite, this process repeatability is sufficient. For other applications, such as for very rapid thermal processing as described in the above referenced in patent application PCT/US00/41492, thermal treatment to better than 1% is needed.
Prior literature has described the concept of optically sensing the plasma jet position from its light emission and feeding back the sensed position to magnetic fields applied to the plasma jet over a single localized area to stabilize and to re-adjust the plasma jet position. Localized magnetic fields applied to a localized area of the plasma jet have been described in open literature and patent disclosures. A three magnetic pole geometry is described in the International patent WO9212610 by Pavlovich et al xe2x80x9cDevice for plasma-arc processing of material.xe2x80x9d Four magnetic pole configurations are described in the Russian patent RU2059344 by Siniaguine xe2x80x9cPlasma current generating device;xe2x80x9d in the International patent WO9746056 by Siniaguine xe2x80x9cApparatus for generating and deflecting a plasma jet;xe2x80x9d and WO9831038 by Siniaguine xe2x80x9cPlasma generation and plasma processing of materials.xe2x80x9d In these configurations, the position of each leg of the plasma jet is sensed at one location and a steering magnetic field is applied at one location.
The magnetic fields are generated by electromagnets. Changing the electrical current to the coils in the magnetic circuits varies the applied magnetic field strength. Response time to a sensed change in the path position to magnetically correcting the path position can be fast. Feed back loops with response time near 1 msec have been demonstrated. However, these magnetic field configurations are limited in their capability to precisely control the plasma jet position with consequent limitation in control of the treatment of the substrate.
The prior art of sensing the plasma jet path position at a single point and feed-back control to the magnetic field applied to one localized position cannot precisely control the plasma jet position and direction in the vertex region where the hot gas stream is generated. The difficulty arises from variations in the direction of the plasma jet path as it emerges from the electrode assemblies 22 and 24. Particularly for the cathode assembly 24, the path direction of the emerging plasma jet varies from one position to another within a cone concurrent with the longitudinal axis of the electrode assembly. It is believed this variation in the path direction originates from a variation in the position of the electron emission spot on the emitting surface within the electrode assembly.
The prior art optically senses the position of each leg 21.1 (PJ1) and 21.2 (PJ2) of the plasma jet 20 and to feed-back any deviation from a pre-set position to a localized magnetic field whose strength and direction may be varied to bring the plasma jet position back to the pre-set position at the optical sensing point. Optical sensing of the path position at a given location is conveniently done by using two optical sensors, S1 and S2, at different positions that are aligned to pick-up the emitted light from that given location on the plasma jet path. The optical signal is fed from each sensor position to a photo-sensitive array such as contained by a CCD camera. The electrical current output from the pixels in the photo-sensitive array along with the geometry of the positioning of two optical sensors, which view the plasma jet position, can then be used to determine the position of the plasma jet at the given, sensed location.
Magnetic field configurations using three and four magnetic poles, referenced above, apply a localized magnetic field that can be varied in intensity and direction in the two directions perpendicular to the plasma jet path direction. The current in the plasma jet then responds to a force by the applied magnetic field according to the well-known relation:
dF=I(d/xc3x97B)xe2x80x83xe2x80x83(2)
where: dF is the force that results from an applied magnetic field, B, on the current element of length d/ (i.e., plasma jet element of length d/) through which a current, i is flowing. The direction of dF is perpendicular to both the directions of the current, i, and the applied magnetic field, B. dF, d/ and B are vectors. xe2x80x9cxxe2x80x9d denotes the standard vector cross-product that applies the component of B that is perpendicular to d/.
According to (2), the plasma jet path direction will change in a direction perpendicular to the applied magnetic field direction. Various design configurations for the magnetic poles that apply the localized magnetic field may be used. The common feature of each must be that the field direction and intensity be variable in two directions perpendicular to the plasma jet path direction.
FIG. 2 shows how use of the prior art for correction of a variation in the plasma jet path cannot provide a precise correction to the position and the direction of the plasma jet at the vertex where the hot gas stream is generated. The heavy dashed line 23.1 for the path of plasma jet leg PJ1 is a path with no deviation as shown in FIG. 2. The solid shaded line 21.1 for PJ1 is a path for which the plasma jet emerges from the electrode assembly at an angle to the normal or non-deviated path.
In the prior art, a magnetic field is applied to each leg of the plasma jet at a single localized area. In FIG. 2, the components of the applied magnetic field perpendicular to the path direction are shown in a plane M for the plasma jet leg PJ1. The path position for each leg of the plasma jet is sensed at a single position. In FIG. 2 the position of PJ1 is sensed at position P. Two optical sensors S1 and S2 look at position P from two directions to provide the information needed to determine the 3-dimensional position of the leg of the plasma jet PJ1 at P. This position is fed-back to a magnetic control system, not shown in FIG. 2, that applies magnetic fields that can be varied in intensity and direction at the localized position M.
If the optical sensing system detects that the plasma jet position has changed from a desired position, the magnetic field is changed to bring the sensed path position back to the controlled position at P (see the dashed path 23.1 for PJ1). As shown for the deviated path in FIG. 2, the localized magnetic field applied at M is changed to bring PJ1 to its pre-set position at P. To make this correction, the magnetic field intensity in the direction perpendicular to the plane of the paper would be changed. However, as shown by FIG. 2, even though the plasma jet path 21.1 is at the correct position at the sensing position P, the direction, i.e. its vector, and the position of the path of PJ1 at the vertex 25 is changed to a less desired position as represented by the dashed path 23.1 at the vertex 25.
Consequently, there is a variance in the hot gas stream generated at the vertex. FIG. 2 shows a 1-dimensional variation in the plasma jet path. In practice the path variation and the magnetic field feedback control is in 2-dimensions. The variation in the position and direction of the plasma jet after correction of a variation can be reduced by making the sensed path position P close to the vertex 25 but optical interference from the second plasma jet PJ2 limits how close P may be to vertex 25. In practice the best positioning attainable with single position sensing as employed by the prior art yields a process stability control that is of the order of about 5%. For many substrate processing steps, such as the rapid thermal processing as described in the aforementioned patent applications by Bollinger et al, this stability is inadequate.
This invention discloses a plasma jet generator, using a magnetic field system, including optical position sensing, that gives precise control, typically better than about 1%, with 0.1% being obtainable, either over the position of the plasma jet at the vertex or with the processing steps conducted with the hot gas stream obtained from the plasma jet. Hence, and in particular, such control relates to the use of a hot gas stream that subsequently processes a substrate. For precise control over the plasma jet legs in accordance with the invention a means is provided with which a precise control over the position of the plasma jet in the region where the hot gas stream, that processes the substrate, originates.
This is achieved in accordance with one feature of the invention by using multiple plasma jet path sensing positions and multiple localized steering magnetic fields for each plasma jet leg so that their positions as well as their directions in the vertex zone, where the hot gas stream is generated, are precisely controlled. With a plasma jet generator system in accordance with the invention one can virtually eliminate plasma jet path instability at the vertex region.
This invention solves the problem of the limitation on treatment uniformity resulting from lack of precise control of the plasma jet direction and position in the region where the hot gas stream is generated. This disclosure recognizes that this lack of control:
1. Arises from the variation of the plasma jet path as it emerges from the electrode assemblies.
2. That prior art control configurations, consisting of sensing the path position at a single position and changing the path position by applying a localized magnetic field at a single position, are limited in capability to correct resulting plasma jet path deviations.
In accordance with one embodiment of this invention, the position of both the arms of the plasma jet are each optically sensed at two or more positions, and, magnetic fields are applied at separate areas to each leg of the plasma jet to allow precise positioning of the two legs of the plasma jet where they meet and generate the hot gas stream.
In accordance with another embodiment of the invention only the position of the cathode originating arm or leg of the plasma jet is precisely controlled with a second optical sensor and magnetic field control because the other anode arm is more stable.
In still another form of the invention, the voltage between the cathode and anode assemblies is monitored and variations of the voltage are attributed to a change in the position of a plasma jet leg and thus used to adjust the plasma leg position with a second magnetic field.
It is, therefore, an object of the invention to provide a plasma jet generator and a method with which a precise control over the plasma jet position at a vertex where a hot substrate processing gas stream is produced is obtained.
It is a further object of the invention to produce a method and apparatus with which a highly improved control, typically in the range from about five to 10 to one or better, over the processing of a substrate with a hot gas stream is obtained.
These and other objects and advantages of the invention can be understood from the following detailed description of several embodiments in conjunction with the drawings.